A closely related document is another, coowned U.S. utility-patent application filed in the United States Patent and Trademark Office substantially contemporaneously with this documentxe2x80x94and also hereby incorporated by reference in its entirety into this document. It is in the names of Askeland et al., entitled xe2x80x9cADAPTIVE INCREMENTAL-PRINTING MODE THAT MAXIMIZES THROUGHPUT BY SHIFTING DATA TO PRINT WITH PHYSICALLY UNALIGNED NOZZLESxe2x80x9d xe2x80x94subsequent assigned utility-patent application Ser. No. 09/492,929, filed Jan. 27, 2000 and now abandoned.
This invention relates generally to machines and procedures for incremental printing of text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to alignment provisions in a scanning machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. The invention employs threshold printmode techniques to optimize image quality vs. operating time.
The foregoing statement of the field of the invention speaks of xe2x80x9cthresholdxe2x80x9d techniques in the printmode area. This term is used merely to emphasize that the printmode techniques required for practice of the present invention are extremely basicxe2x80x94almost primitive, in comparison with the very sophisticated present state of the printmode or printmasking art.
For example, the related-art section of U.S. Pat. No. 5,677,716 of Cleveland, filed seven years ago, discusses space-rotation, sweep-rotation and autorotation of masks. More-recently filed patent documents introduce manual pseudorandomization, xe2x80x9cneighborhoodxe2x80x9d conditions, balanced randomization and determinism, automatic generation, and real-time generation of printmasksxe2x80x94including columnwise mask generation within each swath, xe2x80x9cprecooked masksxe2x80x9d and xe2x80x9cpop-upxe2x80x9d masks.
Although entirely compatible with all of the advanced techniques just noted, the present invention by comparison actually needs only the most simple or pedestrian tools of the printmode artxe2x80x94little more, in fact, than the concept of a printmask height. Therefore, as will be seen, these techniques are indeed at the threshold of the print-mode arena; yet their use in the present invention confers potent advantages.
(a) The need for registrationxe2x80x94Many incremental printers use more than one array of multiple printing elements, such as for example multipen inkjet printers. Most often the different arrays (xe2x80x9cpensxe2x80x9d or xe2x80x9cprintheadsxe2x80x9d) print in different colors, including black; however, in certain cases some of the different arrays print in different dilutions or saturations of common colors. Other uses of plural arrays may occur.
In such printers it is necessary that markings made by the different arrays be in register with one another. At least markings should be adequately registered to prevent a human viewer from seeingxe2x80x94with the unaided eyexe2x80x94the effects of misregistration.
In general, however, such systems are subject to mutual misalignment of the arrays and therefore misregistration of the markings. Various different kinds of provisions are known for reducing such misalignment to the point at which registration at least satisfies visual requirements.
(b) Tight tolerancesxe2x80x94For convenience and definiteness such provisions can be described with reference to multipen inkjet printers, and more particularly thermal-inkjet units of the Hewlett Packard Companyxe2x80x94such as the models known as PaintJet(copyright), DeskJet(copyright) 1200C and HP(copyright) 2000C. In some of these printers, alignment is achieved mechanically: the pens are aligned to within a dot row, by virtue of individual machining to achieve fine mechanical tolerances.
As a result these printers can print with all their nozzles, and with straightforward good alignment along the direction of printing-medium advance. This is true for the 7 dot/mm (180 dot/inch or xe2x80x9cdpixe2x80x9d) PaintJet printer, and for the 12 dot/mm (300 dpi) model 1200C printer.
It will be understood, however, that this solution to good registration is achieved only at very great cost, since the machining required is time consuming and costly. Furthermore, this solution is commercially feasible only because spacing of the dot rows at {fraction (1/7)} or {fraction (1/12)} mm is relatively coarse in terms of machine-tooling standards.
(c) Reserved elementsxe2x80x94In the HP 2000C, resolution is 24 dot/mm (600 dpi) and at such fine spacings mechanical machining begins to be an uneconomic way to achieve registration. Instead, this printer uses a pen alignment scheme that reserves eight nozzles of each pen for pen-to-pen alignment.
In that system, nominally four nozzles 13K, 15K (FIG. 1) are reserved at each end of the black-ink pen K, and only the nozzles 14K between those two end zones are employed to print. Fixed upper and lower limits 11, 12 are established, reflecting these assumed reservationsxe2x80x94and these fixed limits 11, 12 are applied to the color pens C, M, Y as well as the black pen K.
With drift away from the nominal mechanical alignment, however, for example a particular pen, e. g. the cyan pen C as shown, might be two pixel rows lower than its nominal position, and this would require use of two nozzles from the top nominal end zone. This would leave only two nozzles 13C actually reserved at the top, while in exchange the system would give up two nozzles just above the bottom nominal end zone, leaving unused six nozzles 15C as the bottom end zone. The central group of used nozzles 14C still has the same size in nozzles, but these used elements are shifted upward along the nozzle array by a distance equal to two nozzle spacingsxe2x80x94which most commonly (though not necessarily) is two pixel rows.
In practice any such possibility may occur for any of the pens, including the black pen K or the magenta or yellow pen M, Y rather than the cyan pen C. The particular pattern illustrated, with the magenta pen M shifted upward about six rows and the yellow pen Y at the nominal position, is purely exemplary.
In any event, eight nozzles are always sacrificed to the needs of alignment, and in practice the limit lines 11, 12 are fixed in position relative to the world, or in other words to the pen carriage. The total number of nozzles in each pen is 304; therefore the maximum number of these printing elements that fall within the central regions 15 and can be used is 296.
The remaining 8/304 or about 2.7 percent of the nozzle complement is abandoned at the outset. (Of course different reserved and total numbers of nozzles may be present in different models or from different manufacturers; these values are simply exemplary.) From these numbers it can be seen that the end zones 13, 15 of all the pens have been drawn greatly exaggerated relative to the corresponding central regions 14, simply to facilitate clear discussion.
To a person not skilled in this field, such a seemingly small fraction might not appear significant. In this extremely competitive commercial environment, however, these numbers represent a major handicapxe2x80x94for two reasons.
(d) Direct costxe2x80x94First, the provision of eight additional nozzles is far from a small matter. Each nozzle in the nozzle plate is not merely a hole in an amorphous structure, but to the contrary must be accompanied by ink-provision and firing components within the printheadxe2x80x94and these facilities are fashioned in what is the equivalent of a multilayer silicon circuit chip.
The cost of printed-circuits and the like formed in silicon is notoriously expensive, to the extent that common slang in industry refers to silicon xe2x80x9creal estatexe2x80x9d. In addition, the cost of expanding the amount of space used in such a structure is not even linear: the bigger the chip, the more expensive per unit area because of the greater difficulty of working larger chips and the progressively escalating scrap factor.
(e) Hidden costsxe2x80x94The first major handicap of cost does not end there, however, for the silicon real estate carries its operating environment along with it in an ever-widening series of ripple effects. The connecting circuitry must have more contacts, and the print zone must be therefore longer and more expensive.
So must the printer chassisxe2x80x94and accordingly the shipping box. In turn the space and therefore the cost of shipment and inventory storage are implicated as well.
(f) Performance: speedxe2x80x94The second major handicap, even more severe, is reduced throughput. Since the marking-element (e. g. nozzle) array height is in a sense artificially restricted, each swath of marks is likewise restricted in height. A greater number of such swaths is therefore needed to cover any given image height (for instance, a full page) on the printing medium.
Each swath is marked in a single respective pass of the marking-element array (xe2x80x9cpenxe2x80x9d, or printhead) across the sheet of printing medium. The amount of time required to make each pass the marking-element array across the sheet is essentially independent of the array height.
Therefore the restricted array height can be translated into a greater number of passes per image or page, and a longer time to make those passes. In short, the number of minutes per page increases and the manufacturer""s advertised page-per-minute performance figure falls.
The significance of this is well known to anyone who has ever glanced at a display of competing printers in a retail store. The number of pages printed per minute or hour is one of the two or three most heavily emphasized and most conspicuous characteristics of a desktop computer printer.
While a 2xc2xd% shift alone may not be visible in the published information, naturally several such factors from different causes do become visible and significant. Hence the importance of the eight unused nozzles in terms of performance.
The actual impact of a 2.7% loss of nozzle complement is likely to be extremely nonlinear. For example, in a two-pass printmode if a system comes up just four nozzles short, two additional entire passes must be made to complete the pattern.
At the other extreme, if a system in a two-pass mode happens to reach the end of a pattern and only needs, say, half the nozzles in the printhead for the final swath, then loss or gain of four nozzles might be entirely inconsequential. Since neither of these extreme kinds of circumstances can be predicted generally in advance, it can only be said that the number of available nozzles is very important.
In many practical cases, just a few nozzles more or less can assume a throughput importance out of all proportion to their number. This is surely true in human terms, taking into account the engineering effort regularly devoted even to merely trying to evaluate that importance.
When an overall printhead is just one-half inch tall (including any nozzles that are candidates for reservation) and the image is an integral multiple of a half inch, loss of only two or three nozzles can regularly trigger a requirement for additional passes. This is an adverse case which occurs with particular frequencyxe2x80x94because half-inch nozzle arrays are currently in favor, and many desired images are in integral-inch (e. g., eight by ten) sizes.
(g) Performance: lifexe2x80x94A related phenomenon is the effect on useful life of each printheadxe2x80x94now the number of pages that can be printed before the head wears out. This is particularly important for printheads that are refillable after the ink supply is exhausted.
When the number of swaths per page rises, the number of rows on each page that must be printed by each nozzle also rises. This means that a greater part of each nozzle""s useful life must be expended on each page, and so lowers that life in terms of number of pages.
When several nozzles are worn out, the entire head must be discarded. Hence a 2xc2xd% reduction in nozzle complement translates directly into a 2xc2xd% reduction in pen life.
(h) Statistical wastexe2x80x94The reserved-nozzle approach to alignment is ingenious and extremely useful in comparison with machining to better than 1/24 mm. Nevertheless, as has now been shown, that approach is squeezed by two pressures: performance penalties suffered by failing to use all the silicon real estate (and its supporting installed environment) that is actually in place and paid for, and high mechanical cost of adding more capacity to compensate.
If either of these is allowed to control, however, then alignment suffers. This is a particularly acute aggravation because statistically, given the nature of tolerances and departure from nominal conditions, usually the most likely condition is the nominal onexe2x80x94i. e., the used nozzles being the 296 in the middle.
The least likely condition is the extreme case of the used nozzles extending all the way to one or the other end of the array. The intermediate two cases are in general of intermediate likelihood.
Hence the major fraction of the eight-nozzle sacrifice outlined above is made on behalf of a relatively minor fraction of the actually occurring cases of misalignment. To put it another way, the minimum nozzle loss is 8/304 or 2.7 percentxe2x80x94regardless of the actual amount of mechanical misalignment.
Every user must pay the full penalties outlined above, even though the printer owned by a representative user has an actual need for perhaps less than half of those penalties.
(i) Conclusionxe2x80x94These registration problems have continued to impede achievement of uniformly excellent incremental printingxe2x80x94at high throughput and very low costxe2x80x94on all industrially important printing media. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
In preferred embodiments of its first major independent facet or aspect, the invention is a method of printing with plural pens. Each pen has multiple nozzles, and the pens in general are not perfectly aligned.
The method includes the step of determining pen-to-pen mechanical misalignment. It also includes the step of automatically ascertaining the maximum number of nozzles that can be used while printing (1) with the used nozzles of all pens substantially aligned, and (2) substantially without relative shift of respective data for the plural pens. This ascertaining step is based upon the determined misalignment.
The method also includes the step of automatically printing with the ascertained maximum number of nozzles. The foregoing may represent a description or definition of the first aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, the invention makes it possible to break out of the difficult dilemma of trading off pen-alignment cost against throughput. The invention enables use of the more-modern solution of providing extra nozzles to avoid fine mechanical tolerancesxe2x80x94but without necessarily having to sacrifice all those nozzles all of the time.
Thus some or all of the extra nozzles are recaptured for use when the pens happen to be mutually aligned to better than plus-or-minus four nozzles from their average positionxe2x80x94and this is so in the great bulk of actual cases. This invention also avoids use of data shifting, and thereby of the artifacts that arise in that approach when using certain kinds of printing media.
Although the first major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably if the determining step establishes that the pens are aligned within a dot row, then the ascertaining step ascertains that all the nozzles can be used, and the printing step prints with all the nozzles. Another preference is that the printing step include automatically employing a printmode that specifically accommodates the specific ascertained maximum number of nozzles.
Yet another preference is that each pen have a few hundred nozzles, all the pens be in mechanical alignment within a few nozzle spacings, and the ascertaining step include eliminating from printing use only at most a few nozzles that are outside said mechanical alignment. In this case, it is further preferable that the eliminated few nozzles, of the few hundred nozzles in each pen, most typically amount to roughly one percent of all the nozzles. If this is so, then it is yet further preferred that the maximum possible number of nozzles eliminated be roughly three percent of all the nozzles.
In preferred embodiments of its second major independent facet or aspect, the invention is a printer. The printer includes plural printheads each having a multiplicity of marking elements, each element being subject to deterioration and shortening of operable life through use.
The printer also includes some means for extending the life of the marking elements, and thereby the life of the printheads, by distributing use of the marking elements over a maximum number of marking elements. For purposes of generality and breadth in discussion of the invention these means will be called simply the xe2x80x9clife-extending meansxe2x80x9d.
The life-extending means in turn include some means for printing with the maximum number of nozzles that can be used while printing with the used nozzles of all pens substantially aligned and substantially without relative shift of respective data for the plural pens. These means, again for breadth and generality, will be called the xe2x80x9cprinting meansxe2x80x9d. The alignment which is a characteristic of the printing means is based on known pen-to-pen mechanical misalignment.
The foregoing may represent a description or definition of the second aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention mitigates the ancillary problem of pen-life curtailment through diversion of usable nozzle-plate area into alignment tolerances. It minimizes this problem without incurring a penalty for finer mechanical tolerances.
Although the second major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the life-extending means also include means for automatically establishing the mechanical misalignmentxe2x80x94again here, the xe2x80x9calignment-establishing meansxe2x80x9d.
In this case, preferably the alignment-establishing means include alignment data encoded on the pens, and some means for reading the encoded data. An alternative is that the alignment-establishing means include some means for using the pens to print a test pattern, and some means for reading the test pattern to establish the pen alignment from the pattern.
In the latter situation, it is still further preferable that the pen-using means include portions of a processor programmed to control the pens to print the test pattern; and that the pattern-reading means include a line sensor disposed for sensing the test pattern, and portions of a processor programmed to control the line sensor to sense the test pattern.
Another alternative is that the alignment-establishing means include some means for using the pens to eject test drops, and some means for sensing the test drops to establish the pen alignment from that sensing step. If this alternative is adopted, then a further preference is that the pen-using means include portions of a processor programmed to control the pens to eject test drops; and the test-drop pattern sensing means include a shutter and a detector disposed in combination for detecting drops ejected from particular nozzles of the pen. In this case a preferred detector includes an optical drop detector.
In preferred embodiments of its third major independent facet or aspect, the invention is a printer. It includes plural pens, each pen having multiple nozzles. The pens in general are not perfectly aligned.
The printer also includes some means for determining pen-to-pen physical misalignmentxe2x80x94the xe2x80x9cmisalignment determining meansxe2x80x9d. It also includes portions of a processor programmed to automatically perform these two functions:
ascertain, based on the determined misalignment, the maximum number of nozzles that can be used while printing with the used nozzles of all pens aligned, and substantially without relative shift of respective data for the plural pens, and
control the plural pens to print with substantially the ascertained maximum number of nozzles in all of the plural pens.
The foregoing may represent a description or definition of the third aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this third aspect of the invention provides in an apparatus context the benefits of the method set forth in the first aspect. Without resorting to data shift and the hue-artifact problems that can accompany that approach with some media or at relatively large offsets, the third aspect of the invention maximizes the use of available nozzles.
Although the third major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the processor is programmed to automatically select and put into operation a printmode that substantially fully employs the ascertained maximum number of nozzles in all of the plural pens. Another preference is that the physical misalignment include relative displacement of the pens toward lower or higher positions; and the maximum number of nozzles include all the nozzles, on all the pens, that are between the top end of the lowest pen and the bottom end of the highest pen, inclusive.
Still another preference is that the determining means include alignment data encoded on the pens; and means for reading the encoded data. In this casexe2x80x94if the printer also includes a carriage for holding the pens, and mechanical datum points on the carriage for controlling positioning of the pens relative to the carriagexe2x80x94then it is also preferable that the determining means further include some means for taking into account positional variation in the mechanical datum points.
Preferably too, the processor portions include some means for printing with all the nozzles if the determining means establish that the pens are aligned within a dot row. Other preferences described for the first and second main facets of the invention are also applicable for this third primary aspect.